The best guide for Section II, and the GAMSAT in general, is still the info booklet from ACER. Their 2013 guide is already available as a free download on their website.

Just my two cents though, you should take the following into consideration when preparing for Section 2:

1. You are expected to deal with socio-cultural issues in Task A and personal and social issues in Task B.
2. Timing is a major factor in doing well in this section. You should address any timing issues early on in your preparation.
3. It helps to adopt a structure because this gives you a clear sense of direction given the random topics (I mean, we never know what topics and quotations will come out in the exam) and the limited time.
4. Read a lot of editorials and professional blogs to help widen your exposure to varied topics and issues.
5. Writing a daily journal is also a great way to develop spontaneity of thoughts and a more fluid writing style.

*Note: to be up-front though, I am associated with the Gold Standard company but my advice above is just a general "guide." Of course, if someone had to ask me about which product to recommend, you would know my answer. =)